1. Field of the invention
The invention relates to a device for displaying colour images.
2. Description of Related Art
In devices for displaying colour images, the colour displayed at each point is generally obtained through the combination by additive synthesis of several primary colours. The weighting of the various colours in the combination determines the colour displayed.
Combination by additive synthesis of the primary colours may take varied forms. It may be carried out by the superposition of various beams which each correspond to a primary colour. According to another solution, used for example in mono-imager projectors and back-projectors (which use a single imager or modulator to process the whole set of primary colours), the combination is obtained through the very fast succession of the displaying of each primary colour, that the eye integrates into a resultant colour (sequential procedure).
Conventionally, three primary colours are used. In most display systems these three primary colours are red, green and blue. Through the combination of these three colours with variable weightings, it is possible to obtain at each point a wide range of colours, which describes according to the conventional chromaticity representations (CIE type) the area of a triangle whose vertices represent the primary colours.
In display devices which use such a solution, the set of colours that can be displayed is therefore limited and determined during design as a function of the primary colours used. For example, in a projector or a back-projector whose primary colours are obtained through the successive passage of coloured filters in front of a white light source, the colours of the filters fix the vertices of the triangle whose surface corresponds to the set of colours that can be displayed by the projector or back-projector.
In order to improve the brightness of such a display device, patent application EP 0 749 250 has proposed that the saturation of the primary colours be varied by radial displacement of a specific coloured wheel with respect to the light beam. This coloured wheel presents saturated colours at the centre and becomes progressively desaturated at the periphery. The user can thus alter the brightness at will and, in an inverse manner, the saturation of the images displayed.
However, this solution only very specifically extends the possibilities of obtaining colours by the display device. In particular, the effect is necessarily identical over the whole set of primary colours. Moreover, the result obtained is comparable with conventional brightness adjustment.
In order to compensate for the degradation of the filters over time, patent application WO 95/11 572 proposes that the intensity of the light source be rendered adjustable independently for each filter. This adjustment thus makes it possible to modify as in the previous solution the saturation of the primary colours, but also leaves the hue of each unchanged, thereby limiting the possibilities of adjustment. Moreover, the variation in the intensity at the frequency of passage of the filters renders the power supply system of the lamp complex.